A namespace identifier is not an URL, it is a URI - a very important distinction! As such it does not need to be related to an actually existing domain (although that may sometimes make sense), nor can it be verified in any way by making a network call. A namespace serves to uniquely identify element names, so that it's possible to have XML documents like this:

and it's always clear which "foo" element is meant - the two may be very different in meaning!